1. Field Of The Invention
The present invention relates to surgical cutting instruments and, more specifically, to surgical cutting instruments formed of coaxial inner and outer members and having teeth for cutting tissue, including bone. The invention has particular utility in rotatably driven cutting instruments used in arthroscopic surgery.
2. Description Of The Prior Art
It is known in the prior art to provide surgical cutting instruments comprising an outer sleeve with a cutting aperture and a toothed inner tube rotatably mounted within the outer sleeve, permitting the teeth to rotate past the cutting aperture to cut tissue received therein. Examples of cutting instruments of this type are disclosed in U.S. Pat. Nos. 4,368,734 (Banko) and 3,618,611 (Urban). Such instruments possess numerous disadvantages including the relative ineffectiveness of the concavely curved sides of their cutting teeth in cutting tissue. Specifically, when the inner tube is rotated relative to the outer sleeve, the crests of the teeth pierce tissue positioned in the cutting aperture and, as the concave sides of the teeth continue to pass through the tissue, the tissue collects or bunches up in the curved spaces between the teeth. Consequently, the concave-sided teeth cut tissue in a tearing or digging action, rather than with a clean shear cut, the result often being trauma to tissue and the need for relatively long cutting times and high cutting forces to cut even small tissue quantities. Additionally, the height of typical curved cutting teeth is very small in relation to their width and to the diameter of the inner tube, and such shallow teeth are capable of cutting only small increments of tissue for each rotation past the cutting aperture. A further drawback to cutting instruments having teeth with concave sides is that the teeth are formed by overlapping radial machine cuts in the inner tube such that the distance between the origins of the radii of the machine cuts is less than the diameter of their corresponding circles. The overlapping radial machine cuts remove substantial quantities of material from the inner tube, resulting in a substantial reduction in the tube strength. The concavely curved cutting teeth, therefore, generally lack the structural integrity required to apply high cutting forces required to cut bone, or to tolerate impact forces arising from incidental contact with other instruments at the surgical site.
Other surgical cutting instruments of the general type described but having straight-sided cutting teeth are disclosed in U.S. Pat. No. 2,721,555 (Jenny) and in an article by Aksenova et al entitled "New Biopsy Instruments", published in Biomedical Engineering (Vol. 13, No. 4). However, these instruments are generally limited to cutting small fragments of easily penetrable tissue, such as skin and other delicate tissue, due to the cutting teeth being formed as very fine serrations. More particularly, the height of the straight-sided cutting teeth is typically very small in relation to their width and to the diameter of the inner tube and, therefore, the effective cutting length of the cutting edge presented by the straight-sided cutting teeth is only slightly larger than the axial length of the inner member segment along which the teeth are defined. Accordingly, the straight-sided teeth generally cut relatively small amounts of tissue for each rotation past the cutting aperture, require relatively long cutting times to cut tissue, and are defined in a relatively weak structure so as to be precluded from use in surgical procedures necessitating high cutting forces that might break or damage the relatively fragile inner member.
The structural configuration, limited cutting effectiveness and minimal strength of presently available curved and straight-sided cutting teeth typically limit instruments utilizing these cutting teeth to specific cutting functions and types of tissue. Such instruments cannot be employed, for example, to perform multiple, diverse cutting functions, such as resecting, trimming, side cutting, whiskering, burring and the like, on diverse types of tissue including relatively elastic tissue as well as relatively hard tissue, such as bone. The functional limitations of such instruments make it necessary to utilize many specialized cutting instruments in surgical procedures, such as arthrosopy, involving multiple, diverse cutting operations on different types of tissue, thereby adding significantly to the time and complexity of these procedures.
A further drawback shared by surgical cutting instruments of the type described is that a gap is usually maintained between the outer sleeve and the inner tube to permit the inner tube to freely rotate within the outer sleeve without friction and the attendant and potentially damaging heat. This gap, however, is typically wide enough adjacent the cutting teeth for tissue to be drawn therein. Tissue is thusly caught between the outer sleeve and the rotating inner tube and, when the inner tube is rotated at high speeds, the tissue becomes wrapped around the inner tube. When this occurs, rotation of the inner tube is impeded and sometimes totally blocked, requiring time and attention to be diverted from the surgical procedure to free the caught tissue. Moreover, the gap between the inner and outer tubes tends to promote misalignment between these tubes when cutting forces are imposed between them. Since there is no structure in the gap to preserve positional stability of the inner tube, the resulting misalignment impairs the cutting efficiency and effectiveness of the surgical instrument.